Reproduction processes are known wherein positive-working and negative-working photopolymerizable elements are exposed imagewise through a transparency forming nontacky and tacky image areas. Positive-working photopolymerizable elements are described in Chu and Cohen, U.S. Pat. No. 3,649,268, and negative-working photosensitive elements are described in Chu and Fan, U.S. Pat. Nos. 4,174,216 and 4,191,572. The image is developed by toning with a suitable toner which desirably adheres only in the tacky image areas. Excess toner which may be present is removed from the nontacky image areas to provide, for example, an image which is a proof of the original or which can be used to transfer the image to another surface. Multilayer proofs such as surprint proofs can be made as well. The process for preparation of such proofs is well known in the graphic arts, and is described in detail, for example, in U.S. Pat. Nos. 3,649,268 and 4,174,216, which are hereby incorporated by reference.
Various nonelectroscopic toners and applicators for applying toners have been developed. Typical nonelectroscopic toners are described in Chu and Manger, U.S. Pat. No. 3,620,726, Fickes, U.S. Pat. No. 4,397,941, and Matrick U.S. Pat. Nos. 4,565,773 and 4,546,072. A wide variety of useful resin matrices, such as, polyvinyl chloride, cellulose acetate, cellulose acetate butyrate, polystyrene, and polymethyl methacrylate are mentioned. These resins are polymers which can be characterized by their ability to absorb water. Absorption of water is a function of the polarity of the polymer as well as the substituent groups of the polymer. Examples of polymers having high water absorption characteristics are polyvinyl formal which has basic heteroatoms such as oxygen or nitrogen and nylon 66 which has acidic hydrogen atoms. Water-soluble polymers such as polyvinyl alcohol, polyvinylpyrrolidone and methyl cellulose are not suitable for making toners because these polymers are extremely hygroscopic.
Water absorption of polymers in the form of molded plastics is determined by ASTM D 570. The plastic sample is immersed in water for 24 hours, wiped and the weight increase is measured. The sample has little surface area because it is molded. Accordingly, absorption values are lower than absorption values obtained for particulate toner samples in contact with water vapor. Nevertheless, polymers can be ranked according to their ability to absorb water. The Polymer Handbook, 2d ed., Vol. VIII, pages 5-6 (1975) has a table listing polymers according to their ability to absorb water using the method described above. Those polymers in the highest absorption category (0.8%-1.7%) consisted of cellulose derivatives, polyamides with no more than six carbon atoms and polyacetals. The second highest category (0.08%-0.4%) contained polymers such as poly(oxymethylene), poly(methyl methacrylate), melamine and acrylonitrile polymers. Those polymers having the lowest absorption (&lt;0.08%) contained polyolefins and copolymers of polyolefins, polystyrenes and halogenated polymers.
It has been found that the toned density of a proof is variable when the toner is not humidity resistant. The toning process then becomes dependent upon the relative humidity of the area in which toning is taking place. Proof-to-proof uniformity is degraded as well as the overall reliability of the system. The dry, nonelectroscopic toners described in the examples of Chu, Fickes and Matrick contain a resin (i.e., cellulose acetate) which is not humidity resistant. The moisture retaining capacity of cellulose acetate is such that the toned density of the resulting proof varies depending upon the humidity of the surrounding environment.
While automatic toning machines have improved the uniformity and consistency of short-term proof-toproof toning density, it has been found that the flow of toner through the toning apparatus is impeded because the toner is not humidity resistant. The toner particles absorb moisture, aggregate and clog the nozzles of the toning apparatus. Consequently, the desired toned density cannot be achieved. These factors increase the need for toners which perform uniformly and reproducibly over time.
Proofing colors obtained using nonelectroscopic toners also should have improved color purity/decreased grayness. This upgrades the ability of the proofing system to match four color process printing.
Furthermore, it is important that the toner particles have small particle size because smaller particle size allows for higher resolution thereby increasing the potential toning scale. Current proofing systems want to achieve higher resolutions in order to obtain more accurate proofs.
Thus, there is a need for a dry, nonelectroscopic humidity resistant toner for producing high resolution images having higher toned optical density with improved color purity.